Method and device for concentrating substances in solid particle state

ABSTRACT

A method for concentrating a particulate matter, comprising at least two constituents of different densities, in which a slurry of particulate matter is subjected to centrifugation and to centripetal pulses in a centrifugation chamber. A dense fraction of the slurry and a light fraction of the slurry are drawn off separately from the centrifugation chamber. In order to produce the centripetal pulses, a fluid is injected into the slurry, in a direction oblique or tangential to the direction of centrifugation. The fluid is injected substantially continuously into the slurry.

FIELD OF THE INVENTION

The invention relates to the concentrating of materials in solidparticle state, comprising several organic and/or inorganic constituentsof different densities.

The invention relates more particularly to an improved method for thedensimetric concentration of ultrafine particles of such materials,according to the principle of jigging in a centrifugation chamber, andalso to an apparatus for implementing this improved method.

PRIOR ART

Jigging is a well-known technique for concentrating solid materialscomprising substances of different densities [for example, an orestripped of its components (naturally for alluvial and eluvial depositsor after grinding), the cleansing of soil polluted with buckshot, or anyother mixture of different materials].

Jigging involves several physical principles in order to allow thesegregation of particles according to their density while preventing thephenomenon of equivalence which occurs during the freefall of theseparticles, where a coarse light particle has the same sedimentationspeed as a fine heavy particle.

The physical principles are the following:

the displacement of the particles during a short acceleration dependsonly on the density of the particles;

the free sedimentation speed promotes the sedimentation of the coarsestof particles;

the retarded sedimentation (the particles impede one another and collidewith one another during the sedimentation) promotes the sedimentation ofthe finest particles. This phenomenon occurs especially at the end ofsedimentation. It has a tendency to compensate for the second, whichfavors the coarse particles.

Jigging techniques can be divided up into two major families: jiggingtechniques under the action of gravity and jigging techniques bycentrifugation. The techniques that use the action of gravity generallyuse two segregation driving forces, one of which uses the first physicalprinciple stated above and the other of which uses the other twophysical principles. However, as soon as the particle size decreases,the specific surface area increases and the surface forces (drag) becomepredominant with respect to the volumic forces (weight) which are incompetition in the jigging phenomenon. One way to solve this problem isto centrifuge the material in order to increase the volumic forces.Centrifugal jigging techniques rarely use the first segregation drivingforce, since the means normally used to subject the solid particles toshort accelerations are not generally satisfactory and impair thecorrect functioning of the jig. In document WO-90/00090, a centrifugaljigging apparatus is described, in which the two segregation drivingforces are used. To this effect, in this known apparatus, a slurry of aparticulate matter to be concentrated is subjected to centrifugation ina cylindrical chamber, the peripheral wall of which comprises a gridcovered with a filter bed and, during the centrifugation, the filter bedis subjected to pulsed centripetal displacements which have the effectof subjecting the slurry in the chamber to isolated centripetal forces.The combined action of these isolated centripetal forces and of thepermanent centrifugal force on the slurry gradually generates radialstratification of the particles of the matter in the centrifugationchamber, as a function of their respective densities, thisstratification being substantially independent of the sizes of theparticles or influenced very little by the latter. The dense particlescollect in a peripheral zone of the slurry and the less dense particlesconcentrate in a central zone thereof. In the apparatus of documentWO-90/00090, a series of flexible-wall cavities, fed with water,surround the abovementioned grid of the chamber. The chamber, its gridand the cavities are entrained at high speed in order to centrifuge theslurry and the flexible wall of the cavities is subjected todisplacements according to a defined frequency, in order to project thewater that they contain through the grid and to subject the filter bedto centripetal pulses. In this known apparatus, the pulsing transmittedto the slurry is induced by mechanical effect, the disadvantageousresult of which is that the frequency of the pulses is limited becauseof problems of mechanical inertia. However, a very high frequency isnecessary in order to cause the very short accelerations required forthe segregation of very fine particles. The finer the particles are, theshorter these accelerations must be. This is because the finer theseparticles are, the larger the specific surface area, and the greater thedrag. Under these conditions, the period of acceleration during whichthe drag effect can be ignored is very short. Therefore, the higher thefrequency of the successive accelerations, the smaller the influence ofthe drag.

The known apparatus of document WO-90/00090 has the additionaldisadvantage of being complicated to construct. In particular, there areserious difficulties in making it leaktight. In addition, the need for afilter bed on the centrifuge grid constitutes another difficulty,particularly the practical production of a grid with an ultrafine meshsize. This results in an expensive construction and an apparatus that isdifficult to run.

SUMMARY OF THE INVENTION

The invention aims to remedy the drawbacks of the known centrifugalapparatus described above.

The invention aims more particularly to provide a new and improvedmethod for concentrating, by the centrifugal jigging technique,materials in solid particle state, comprising several organic and/orinorganic constituents of different densities.

The invention aims most especially to provide a method which makes itpossible to carry out, simply and economically, rapid and effectiveconcentration of ultrafine particles of such materials.

An objective of the invention is also to provide an apparatus forconcentrating such materials by the centrifugal jigging technique, saidapparatus having a simple, practical and economical design andproviding, moreover, great reliability and a high operating yield.

By convention, in the rest of this specification, the expression“particulate matter” denotes a solid material in the form of particlesof various sizes and shapes, comprising at least two organic and/orinorganic solid constituents. The particulate matter may, for example,comprise an ore, the constituents of which comprise minerals.

The expression “useful substance” denotes a solid or mineral componentthat it is sought to extract in the concentrated state from theparticulate matter, and the expression “unproductive substance” denotesa solid or mineral residual component that it is sought to separate fromthe useful substance(s).

The term “slurry” denotes an aqueous dispersion or suspension of theabovementioned particulate matter in water or another appropriate liquid(organic or inorganic). The liquid selected should have a density belowthat of the particulate matter.

Consequently, the invention relates to a method for concentrating aparticulate matter, comprising at least two constituents of differentdensities, in which a slurry of said particulate matter is subjected tocentrifugation and to centripetal pulses in a centrifugation chamber,and a dense fraction of the slurry and a light fraction of the slurryare drawn off from the centrifugation chamber, the method beingcharacterized in that, in order to produce the centripetal pulses, afluid is injected into the slurry, in a direction which has a componenttangential to the centrifugation.

In the method according to the invention, the function of thecentrifugation is to subject the particles of the particulate matter toa centrifugal acceleration and, as a result, to centrifugal forces whichwill radially classify the particles of the particulate matter accordingto their respective masses. The centrifugation can be carried out by anyappropriate means, for example using a rotary centrifuge. Thecentrifugation is carried out in a centrifugation chamber. The latter isnormally a revolving chamber. It may, for example, be cylindrical,conical or frustoconical. It is not essential for the definition of theinvention and will be explained below.

The speed of the centrifugation will condition the centrifugalacceleration of the slurry and, consequently, the centrifugal forcesacting on the particles of the particulate matter. It is not essentialfor the definition of the invention. All things being otherwise equal,it will condition the productivity of the method and the precision ofthe cutoff between the light fraction and the dense fraction of theparticulate matter. The optimum centrifugation speed will depend onvarious parameters, among which are the density of the or of each usefulsubstance of the particulate matter, the densities of the unproductivesubstances, the particle size distribution of the particulate matter andthe dimensions of the chamber used for the centrifugation. Theseparameters should be determined in each specific case by those skilledin the art, by means of routine laboratory tests or research departmentstudies.

The function of the centripetal pulses is to subject the centrifugedslurry to isolated centripetal forces of short durations, comparable toimpacts, according to a defined frequency.

In accordance with the invention, the centripetal pulses are obtained byobjecting a fluid into the slurry subjected to the centrifugation, thisfluid injection comprising a component tangential to the centrifugation.

The fluid may invariably be a gas or a liquid. It should besubstantially inert with respect to the constituents of the slurry. Inthe case of a liquid, the latter cannot normally be a substance thatdissolves the constituents of the particulate matter. It may be withoutdistinction an organic liquid or an aqueous liquid. Liquids which aremiscible with the liquid of the slurry are especially recommended. Thesame liquid as that of the slurry is advantageously used, water beingpreferred.

The fluid is injected into the slurry in the form of a localized jet,this jet having a component which is tangential to the direction ofrotation of the slurry and to the peripheral wall of the centrifugationchamber. The injection may be strictly tangential to the peripheral wallof the centrifugation chamber. It is preferably oblique, so as to alsohave a radial component.

The injection of the fluid is preferably carried out continuously, witha substantially constant speed and/or a substantially constant flowrate. A continuous injection with a substantially constant injectionspeed is preferred. The tangential injection of the fluid into theslurry generates, in the latter, local centripetal pulses opposite thefluid injection zone. The particles of particulate matter in the slurryare thus subjected to tangential and centripetal isolated accelerationswhich superimpose on the substantially constant centrifugalacceleration. The frequency of the centripetal accelerations to whicheach particle of particulate matter is subjected depends on therotational speed of the slurry in the centrifugation chamber. Thecombination of the centrifugal acceleration and the isolated centripetalaccelerations produces a gradual stratification of the particles of theparticulate matter in the slurry, according to their respectivedensities, the most dense particles migrating to the periphery of theslurry vortex and the less dense particles migrating in the oppositedirection.

The quality of the stratification of the particulate matter particles inthe slurry and, consequently, the particulate matter concentrationyield, will depend on various parameters, among which are the dimensionsof the centrifugation chamber, the flow rate of the slurry and the speedat which it is introduced into the centrifugation chamber, and also theflow rate and the speed of injection of the fluid into the slurry. Theoptimum values of these parameters will also depend on various factors,in particular on the particulate matter treated, on the respectivedensities of the useful substance and of the unproductive substances, onthe particle size distribution of the particulate matter in the slurryand the concentration of the slurry, and also on the densities of theslurry liquid and of the injected fluid. These optimum values shouldconsequently be determined in each specific case by those skilled in theart, by means of routine laboratory tests.

In the method according to the invention, a dense fraction of the slurryand a light fraction are drawn off. The dense fraction is normally drawnoff at the periphery of the centrifuged slurry vortex, generally in adirection tangential to this vortex.

In a specific embodiment of the method according to the invention, thecentrifugation chamber is cylindrical, the slurry is introduced thereinwith a defined speed, tangentially to the peripheral wall of thechamber, and the dense fraction is drawn off tangentially to said wall.

The term “tangentially” is intended to specify that the direction inwhich the slurry is introduced into the chamber and the direction inwhich the dense fraction is drawn off each comprise a component that istangential to the wall of the chamber. These directions may consequentlybe strictly tangential or may be oblique. It is preferable for them tobe strictly tangential or virtually tangential.

The dense fraction is normally drawn off downstream of the introductionof the pulp into the centrifugation chamber, the expressions “upstream”and “downstream” being defined relative to the direction of rotation ofthe slurry vortex in the centrifugation chamber.

In the specific embodiment which has just been described, the lightfraction of the slurry can be drawn off from the centrifugation chamberaxially. It is preferable for it to be drawn off tangentially to theabovementioned peripheral wall of said chamber, downstream of thedrawing off of the dense fraction.

In the specific embodiment which has just been described, the tangentialspeed of introduction of the slurry into the chamber will condition itsrotational speed in the chamber and, consequently, the centrifugalacceleration.

In this specific embodiment, the cylindrical chamber may be horizontal,oblique or vertical. The chamber is preferably substantially vertical.

In the implementation of the method according to the invention, it isnecessary to evacuate from the chamber the fluid which has served togenerate the centripetal pulses in the slurry. This evacuation may becarried out by any appropriate means, generally downstream of thedrawing off of the light fraction.

In an advantageous embodiment of the specific embodiment which has justbeen described, the fluid which serves to produce the centripetal pulsesis injected through the abovementioned peripheral wall of thecentrifugation chamber, substantially over the entire length thereof.

In a variation of implementation of the embodiment described above, atleast one additional drawing off of an additional fraction of slurry iscarried out, this additional drawing off being carried out downstream ofthe drawing off of the dense fraction and upstream of the drawing off ofthe light fraction. In this variation of implementation of theinvention, the content of useful substance of the additional fraction isintermediate between the respective contents of said useful substancein, firstly, the dense fraction and, secondly, the light fraction. Thisvariation of embodiment of the invention thus produces a fractionationof the particulate matter into several fractions with different degreesof enrichment in useful substance. In the rest of this specification,the abovementioned additional drawing off will be referred to as“intermediate drawing off” and the corresponding additional fractionwill be referred to as “intermediate fraction”.

In the variation of implementation of the invention which has just beendescribed, the yield from the concentration with respect to usefulsubstance can be substantially improved by recycling the intermediatefraction into the slurry that is introduced into the centrifugationchamber.

In the method according to the invention and specific embodimentsthereof, the dense fraction constitutes the useful fraction(concentrated with respect to useful substance) or a by-product(enriched in unproductive substances from the particulate matter),according to whether the density of the useful substance is higher thanthose of the unproductive substances or lower than the latter.

The method according to the invention is especially suitable forconcentrating particulate matter with a small particle size, inparticular in the form of particles of diameter less than 800 μm,generally between 1 and 500 μm, the diameter of a particle being, bydefinition, the diameter of a sphere having the same volume as theparticle.

In a specific embodiment of the method according to the invention, whichis especially suitable for such particulate matter, the centrifugationis adjusted so as to subject the slurry to a centrifugal acceleration ofgreater than 3000 m/s² and the injection of the fluid is adjusted suchthat the centripetal pulses have an acceleration substantially between 1and 5 times the abovementioned centrifugal acceleration.

The invention also relates to an apparatus for implementing the methodaccording to the invention, said apparatus comprising a centrifugationchamber, a device for introducing a slurry of the particulate matterinto the centrifugation chamber, a device for generating centripetalpulses in the slurry in the centrifugation chamber, a device for drawingoff a dense fraction of the slurry and a device for drawing off a lightfraction of the slurry; in accordance with the invention, the device forgenerating centripetal pulses in the slurry comprises a duct which opensinto the abovementioned chamber, through a peripheral wall thereof, andwhich communicates with a fluid injection member.

In the apparatus according to the invention, the peripheral wall of thecentrifugation chamber is a wall of revolution. It may have anyappropriate profile. It may, for example, be a cylindrical wall, aconical wall or a frustoconical wall. Cylindrical walls are preferred.The peripheral wall of revolution may be horizontal, vertical oroblique. The wall is preferably substantially vertical.

The device for feeding the centrifugation chamber with the slurrycomprises a duct which opens into the chamber, through its peripheralwall, this duct also communicating with a member for continuousinjection of the slurry. The slurry introduction duct is placedtangentially or obliquely relative to the peripheral wall. It ispreferably substantially tangential with respect to this wall.

The duct for injecting the fluid which serves to generate the pulsesopens out obliquely or tangentially through the peripheral wall of thecentrifugation chamber. It comprises a tangential component which ispreferably in the same direction as the tangential component of the ductfor introducing the slurry. The fluid injection member is advantageouslydesigned such that the injection of the fluid is continuous and at asubstantially constant flow rate and/or speed.

The device for drawing off the dense fraction advantageously comprises aduct which passes through the peripheral wall of the centrifugationchamber and which is oriented so as to have a tangential component inthe same direction as the tangential component of the duct forintroducing the slurry.

The device for drawing off the light fraction preferably comprises aduct which passes through the peripheral wall of the centrifugationchamber, downstream of the duct for drawing off the dense fraction, andwhich is oriented so as to have a tangential component in the samedirection as the tangential component of the duct for introducing theslurry.

In a specific embodiment of the apparatus according to the invention,the centrifugation chamber comprises at least one additional device fordrawing off a fraction of the slurry, said additional drawing-off devicecomprising a duct which passes through the peripheral wall of thecentrifugation chamber, between the ducts for drawing off the densefraction and the light fraction. The additional drawing-off duct isadvantageously similar to the ducts for drawing off the dense and lightfractions. As a variation, the additional drawing-off duct may beconnected to the device for feeding said chamber in order to recyclethereto the fraction drawn off.

In an advantageous embodiment of the apparatus according to theinvention, the duct for injecting the fluid intended to generate thepulses comprises a slit which is made through the peripheral wall of thecentrifugation chamber, over a substantial length of said wall. Theexpression “over a substantial length of the wall of the chamber” isintended to mean a length greater than half the total length of thechamber, generally at least equal to 75% (preferably 80%) of the totallength of the chamber. By definition, the total length of the chamber isthe length of the chamber, from the slurry feed device to the device fordrawing off the light fraction.

The apparatus according to the invention normally comprises a device forevacuating the fluid that has served to generate centripetal pulses inthe slurry. This evacuation device normally comprises a duct which opensout through the peripheral wall of the centrifugation chamber,downstream of the device for drawing off the light fraction. As avariation, it may comprise a duct which passes axially through thedownstream end of the centrifugation chamber.

The method and the apparatus according to the invention have variousapplications. They in particular have an application for concentratingsoils or ores occurring naturally in the granular or pulverulent state,such as, for example, alluvial products. The method and the apparatusaccording to the invention are especially suitable for the enrichmentprocessing of ultrafine ores, in particular for recovering fine residuesfrom milling and for processing minerals recovered from alluvial andeluvial deposits or after milling. The method and the apparatusaccording to the invention have a most particular application forconcentrating gold ores, diamond ores and any other valuable mineral, ofdifferent density to the environment (cassiterite, wolframite, coltan,tourmaline, garnet, chrysoberyl, spinel, zircon, rhodonite, ruby,sapphire, etc.). The method and the apparatus according to the inventionalso have an application for treating polluted soils, for example fortreating sludge from dredging watercourses, polluted with heavy metals,cleansing soils polluted with buckshot, cleansing industrial areaspolluted with organic and/or inorganic solid materials.

BRIEF DESCRIPTION OF THE FIGURES

Particularities and details of the invention will become apparent overthe course of the following description of the attached figures, whichrepresent specific embodiments of the invention.

FIG. 1 shows a perspective view of a first specific embodiment of theapparatus according to the invention;

FIG. 2 shows schematically a detail of the apparatus of FIG. 1, intransverse cross section along the plane II-II of FIG. 1;

FIGS. 3, 4, 5 and 6 are schemes similar to that of FIG. 2, of fourvariations of the detail of FIG. 2;

FIG. 7 is a scheme similar to that of FIG. 2, of an additional variationof the detail of FIG. 2;

FIG. 8 shows a perspective view of another embodiment of the apparatusaccording to the invention;

FIG. 9 shows the apparatus of FIG. 8 in axial section;

FIG. 10 shows an additional embodiment of the apparatus according to theinvention, in axial section; and

FIG. 11 shows a modified embodiment of the apparatus of FIG. 10, inaxial section.

In these figures, the same reference notations generally denote the sameelements.

The figures are not drawn to scale.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The apparatus represented in FIG. 1 comprises a centrifugation chamber11, delimited by a vertical cylindrical side wall 2.

Two ducts 3 open into the bottom of the chamber 11, tangentially to thecylindrical wall 2, at the two ends of a same diameter. The ducts 3serve to introduce a slurry of particulate matter into the chamber 11 soas to subject it therein to a rotation in the direction of the arrow X(FIG. 2).

The chamber 11 is in communication with a narrow vertical duct 4, whichpasses through the wall 2 over approximately its entire height, and theorientation of which is approximately tangential relative to said wall.The duct 4 is oriented so as to introduce a fluid in the direction ofthe arrow X into the chamber 11. The function of the duct 4 will beexplained below.

The chamber 11 is also in communication with a duct 5 close to its upperend and with a duct 6 in an intermediate zone. These two ducts serve todraw off fractions of the slurry treated in the chamber 11.

The ducts 3, 4, 5 and 6 are oriented in such a way as to open into thechamber 11, tangentially relative to its wall 2.

The apparatus of FIG. 1 is intended for implementing the methodaccording to the invention. To this effect, a particulate matter in theform of ultrafine particles is dispersed in water so as to form aslurry. The slurry is introduced into the ducts 3 with a speed that isuniform over time and controlled so as to subject said slurry to rotarycirculation in the chamber 11. Moreover, pressurized water is injectedinto the layer of slurry in the chamber 11, via the duct 4. Theinjection of the water is continuous and at a substantially constantflow rate, thereby causing pulses in the slurry, opposite the duct 4.Under the action of these pulses, the particles of the particulatematter are subjected to isolated tangential and centripetalaccelerations when they pass opposite the duct 4, in the chamber 11.These isolated centripetal accelerations superimpose on the continuousand substantially constant centrifugal acceleration. The magnitude ofthe centripetal accelerations is determined through an appropriatechoice of the flow rate, of the pressure and of the speed of the waterinjected in the duct 4. FIG. 2 shows schematically the combined actionof the continuous centrifugal acceleration and of the isolatedcentripetal accelerations. In this figure, the lines 7 showschematically the lines of circular stream of the slurry subjected tothe centrifugation in the chamber 11 and the lines 8 show schematicallythe lines of stream of the water introduced into the chamber 11 via theduct 4. Under the combined effect of the continuous centrifugalacceleration and of the isolated centripetal accelerations, radialclassification of the particles of solid matter takes place in thechamber 11, according to their respective densities: the most denseparticles (9) migrate to the periphery of the chamber (11), whereas thelight particles (10) migrate to the center of the chamber. The denseparticles are drawn off with liquid from the slurry, via the duct 6, andthe light particles are drawn off with liquid from the slurry, via theduct 5. In the case where the useful substance of the particulate matteris more dense than the unproductive substances of the particulatematter, the fraction of slurry drawn off from the chamber 11 via theduct 6 is the useful fraction, enriched in useful substance, while thefraction drawn off via the duct 5 contains mostly unproductivesubstances.

In the apparatus of FIGS. 1 and 2, the duct 4 should be oriented in sucha way that the flow of water which enters the chamber 11 has a radialcomponent.

FIGS. 3, 4, 5 and 6 show various arrangements of the duct 4, whichproduce this technical function.

In the arrangement of FIG. 3, the duct 4 enters the chamber 11tangentially to its peripheral wall 2. The chamber widens downstream ofthe duct 4.

In the arrangements of FIGS. 4 and 5, the duct 4 enters the cylindricalchamber 11 obliquely and the diameter of said chamber is uniform.

In the arrangement of FIG. 6, the duct 4 enters the chamber 11 obliquelyand said chamber narrows downstream of the duct 4.

In the apparatus shown schematically in FIG. 7, several ducts 6, 6′, 6″open into the chamber 11, through its wall 2. The ducts 6, 6′, 6″ areangularly offset. They serve to draw off fractions of the slurry whichdiffer by virtue of the density of the solid substances that theycontain. Given the direction of rotation X of the slurry in the chamber11, the density of the fractions drawn off decreases from the duct 6(which is closest to the duct for introducing water 4) to the duct 6″(which is the furthest away from the duct 4). This embodiment of theinvention makes it possible to divide the particulate matter up intoseveral fractions having different concentrations of useful substance.The fractions can be recovered separately. As a variation, the lightestfraction 6″ (or each fraction 6′ and 6″) can be recycled such as intothe introduction ducts 3.

In the apparatus represented in FIGS. 8 and 9, the cylindrical chamber11 contains a cylinder 12 with an apertured wall (FIG. 9), the axis ofwhich coincides with that of the chamber 11. The cylinder 12 is mountedon bearings 13, in such a way as to be able to rotate freely in thechamber 11, so as to reduce the head losses in the rotating slurry. As avariation, the cylinder 12 can be driven by an electric motor (notrepresented). The cylinder 12 is extended by a throat 14 which opens tothe outside, after having passed through a corresponding throat 19 ofthe chamber 11.

During the operating of the apparatus of FIGS. 8 and 9, the slurry 15 isintroduced into the chamber 11 via the duct 3, in such a way that itundergoes a centrifugation into said chamber 11. The slurry splits intoa layer 21 against the wall 2 of the chamber 11. Water 16 iscontinuously injected (FIG. 9), via the duct 4 (FIG. 8), into the layerof slurry. The water which has passed through the layer of slurry passesthrough the apertured wall of the cylinder 12 and is evacuated from theapparatus via the throat 14. The light fraction 17 of slurry isrecovered via the annular aperture 5 located downstream of theapparatus, the dense fraction is recovered via the aperture 6 andfractions of intermediate densities are drawn off via apertures 6′, 6″and 6′″ located between the aperture 6 and the aperture 5.

The apparatus shown schematically in FIG. 10 differs from the apparatusof FIGS. 8 and 9 by virtue of the presence of two annular thresholds 18and 22 on the wall 2, in the chamber 11. The two thresholds 18 and 22are placed between the duct 3 (not visible) for introducing the slurry15 and the duct 5 (not visible) for evacuating the light fraction 17.They together form an annular cavity 23, into which the duct 4 (notrepresented, serving to inject the water 16 for the pulses) and the duct6 (not represented, serving to evacuate the dense fraction 20) open.

During the operating of the apparatus of FIG. 10, the dense fraction 12of the slurry is drawn off from the annular cavity 23 and the lightfraction 17 runs over the threshold 18. All things being otherwiseequal, the apparatus of FIG. 10 produces a more precise cutoff betweenthe light particles and the dense particles of the slurry.

In the apparatus of FIG. 11, the chamber 11 comprises a hydrocyclone 24upstream of the threshold 22. The duct 3 for introducing the slurry 15opens into the hydrocyclone 24. During the operating of the apparatus,the slurry passes through the hydrocyclone 24 and migrates to theannular cavity 23. The cyclone 24 serves to separate the particles whichare too fine from the particulate matter, these particles beingevacuated via the axial shaft 25.

1-19. (canceled)
 20. A method for concentrating a particulate matter,comprising at least two constituents of different densities, in which aslurry of said particulate matter is subjected to centrifugation and tocentripetal pulses in a centrifugation chamber having a length and aperipheral wall, and a dense fraction of the slurry and a light fractionof the slurry are drawn off from the centrifugation chamber wherein, inorder to produce the centripetal pulses, a fluid is injected into theslurry, in a direction oblique or tangential to the direction ofcentrifugation, the fluid being injected substantially continuously intothe slurry.
 21. A method as claimed in claim 20 wherein the slurry isintroduced into the chamber nearly tangentially to the peripheral wallof said chamber.
 22. A method as claimed in claim 20, wherein at leastone additional drawing off of an additional fraction of slurry iscarried out, downstream of the drawing off of the dense fraction withrespect to the direction of rotation of the slurry and upstream of thedrawing off of the light fraction with respect to the direction ofrotation of the slurry.
 23. A method as claimed in claim 20, wherein thefluid is injected through the abovementioned peripheral wall,substantially over the entire length of the chamber.
 24. A method asclaimed in claim 20, wherein the abovementioned direction of injectionof the fluid into the slurry has a radial component.
 25. A method asclaimed in claim 23, wherein the abovementioned direction of injectionof the fluid into the slurry has a radial component.
 26. A method asclaimed in claim 1, wherein the fluid is the liquid of the slurry.
 27. Amethod as claimed in claim 23, wherein the fluid is the liquid of theslurry.
 28. A method as claimed in claim 1, wherein the fluid compriseswater.
 29. A method as claimed in claim 27, wherein the fluid compriseswater.
 30. A method as claimed in claim 1, wherein centrifugation isadjusted so as to subject the slurry to a centrifugal acceleration ofgreater than 3000 m/s², and the flow rate of the continuous injection ofthe fluid is adjusted such that the centripetal pulses have anacceleration substantially between 1 and 5 times the abovementionedcentrifugal acceleration.
 31. A method as claimed in claim 29, whereincentrifugation is adjusted so as to subject the slurry to a centrifugalacceleration of greater than 3000 m/s², and the flow rate of thecontinuous injection of the fluid is adjusted such that the centripetalpulses have an acceleration substantially between 1 and 5 times theabovementioned centrifugal acceleration.
 32. A method as claimed inclaim 1, wherein the particulate matter is in the form of particles, thediameter of which is substantially between 1 and 500 μm.
 33. A method asclaimed in claim 31, wherein the particulate matter is in the form ofparticles, the diameter of which is substantially between 1 and 500 μm.34. A method as claimed in claim 1, wherein the particulate mattercomprises an ore.
 35. A method as claimed in claim 33, wherein theparticulate matter comprises an ore.
 36. An apparatus for concentratinga particulate matter, comprising at least two constituents of differentdensities, said apparatus comprising a centrifugation chamber providedwith a peripheral wall extending along a length, a device forintroducing a slurry of said particulate matter into the centrifugationchamber, a device for generating centripetal pulses in the slurry in thecentrifugation chamber, a device for drawing off a dense fraction of theslurry and a device for drawing off a light fraction of the slurry,wherein the device for generating centripetal pulses in the slurrycomprises a duct which opens into the centrifugation chamber through theperipheral wall, tangentially or obliquely with respect to saidperipheral wall, and which communicates with a fluid injection member,said fluid injection member being designed such that the injection ofthe fluid is continuous and at a substantially constant flow rate and/orspeed.
 37. An apparatus as claimed in claim 36, wherein the device forintroducing the slurry comprises a duct which opens into theabovementioned chamber, tangentially to the peripheral wall and whichcommunicates with a member for continuous injection of the slurry. 38.An apparatus as claimed in claim 36, wherein the duct which communicateswith a fluid injection member extends over substantially the entirelength of the peripheral wall of the chamber, downstream of the duct forintroducing the slurry with respect to the direction of the flow.
 39. Anapparatus as claimed in claim 36, wherein the peripheral wall of thecentrifugation chamber is cylindrical.
 40. An apparatus as claimed inclaim 37, wherein the peripheral wall of the centrifugation chamber iscylindrical.
 41. An apparatus as claimed in claim 36, wherein theperipheral wall of the chamber comprises two annular thresholds betweenthe duct for introducing the slurry and the duct for drawing off thelight fraction, the two thresholds together defining an annular cavityinto which, respectively, the duct for introducing the fluid and theduct for drawing off the dense fraction open.
 42. An apparatus asclaimed in claim 36, wherein the centrifugation chamber comprises ahydrocyclone into which the duct for introducing the slurry opens. 43.An apparatus as claimed in claim 37, wherein the centrifugation chambercomprises a hydrocyclone into which the duct for introducing the slurryopens.